What is wrong with this syntax?

Question

If I write

\noindent\vskip 0ex plus 2ex\includegraphics[width={\textwidth plus 2pt minus 2pt}, angle=0]{J2-23-1}

it is correct

However, if I write

\noindent\vskip 0ex plus 2ex\includegraphics[width={\textwidth plus 0.02\textwidth minus 0.02\textwidth}, angle=0]{J2-23-1}

it said "missing number treated as zero".

However, if I included \usepackage[final]{pdfpages} even if I do not use it, the above error message disappeared.

Could anyone tell me what is wrong?

Answer 1

A glue specification is not allowed as width or height in \includegraphics, it does not support flexible dimensions.

In the first case, plus 2pt minus 2pt goes into the output two times:

\documentclass{article}
\usepackage{graphicx}
\begin{document}
  \noindent
  \includegraphics[
    width={\textwidth plus 2pt minus 2pt},
    angle=0
  ]{example-image}
\end{document}

If package calc is used, then its \setlength internally uses a length register for assignment, thus the glue parts are read and consumed, but never used.

Also in the second case, the glue specifications are set as normal text (without package calc): plus 0.02 is text, then \textwidth is a register, which starts an assignment, but the following m is not valid number for a dimen assignment and causes the error message.

-> Avoid glue specifications in the options of \includegraphics. They do not have a purpose and are only causing trouble. Skip (length) registers can be used nevertheless, they are internally coerced to dimen registers ignoring the glue parts. This can be used as workaround, if the glue specifications cannot be avoided (coming from some user macros or whatever):

\includegraphics[width=\glueexpr(\textwidth plus ... minus ...)\relax, ...]{...}